Sadness. Anger. Frustration. The Ottawa Senators are experiencing it all.

"There's no words that can explain what we're feeling," goalie Craig Anderson said.

He wasn't referring to the agony of defeat after a 4-2 loss to the Pittsburgh Penguins on Wednesday, their fifth setback in the team's past seven games.

The Senators were more upset to learn they won't have the defenseman they believe is the best player in the National Hockey League -- and for what could be a very long time.

Erik Karlsson, their 22-year-old reigning Norris Trophy winner, underwent surgery Thursday to repair a 70-percent tear in his Achilles. He will be out three to four months, according to Ottawa GM Bryan Murray."He's playing 30 minutes a game, a Norris Trophy winner, arguably the best player in the League. It's obviously a big loss -- but like every injury, it's somebody's opportunity," coach paul MacLean said Wednesday night. "We have to find out who that's going to be."

During the final minute of the second period, Matt Cooke came from behind to pin Karlsson against the left-wing boards. Cooke's left skate made contact with the back of Karlsson's lower left leg, leaving Karlsson in obvious pain. He immediately headed to the dressing room.

"With (Karlsson), it seemed like everything was going for him," said defense partner Marc Methot, noting Karlsson's six goals and 10 points in 13 games. "All of the sudden, he gets horrible luck out there. It's one of those injuries that requires a lot of bad luck to get. We're going to be missing him huge back there."

There was no penalty called on the play on Cooke, who has a history of supplemental discipline from the NHL. He was suspended for the final 10 regular-season games and the first round of the Stanley Cup Playoffs for an elbow on the New York Rangers' Ryan McDonough in March 2011.

The NHL's Department of Player Safety reviewed the play and spoke to both general managers before determining there would be no supplemental discipline.

When asked if Cooke intended to injure Karlsson on the play, MacLean said: "That's not up to me to decide."

Added Anderson: "I don't know if it's intent to injure, but I don't know why you would hit somebody like that in that situation. I can't tell if it's intent to injure or not, but it obviously catches Erik really unfortunate, a heel of his skate where we don't have any padding.